Syringe shield
This application is a continuation-in-part of applicant&#39;s co-pending U.S. patent application Ser. No. 130,277 now U.S. Pat. No. 4,874,383. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to hypodermic needles, and especially a syringe assembly, and specifically to such assembly that is safe to use in a medical environment. 
     DESCRIPTION OF THE PRIOR ART 
     Typical means for ensuring the sterility of a hypodermic needle prior to use, and for the protection of a handler subsequent to use, include the familiar cap that seats on a collar surrounding the needle. However, these caps do not reliably protect persons handling the syringe after use. There have been numerous instances of injuries and consequent infection caused by users trying to slide a cap back over a needle, and in doing so puncturing their own skins. While in many instances little harm results from such injuries, with the advent of auto-immune disease syndrome (AIDS) medical persons have viewed with greater alarm the ease with which a potentially contaminated needle can puncture the skin of anyone handling the syringe subsequent to the administration of an injection to or the withdrawal of blood from a patient who may have a highly communicable disease. 
     The typical modern syringe is a disposable item having a plunger, barrel and needle with a protective cap over the needle. Many attempts have been made to improve on the conventional needle protection means described above, and perhaps the most relevant known to applicant is disclosed in U.S. Pat. No. 4,356,822 granted on Nov. 2, 1982 (Winstead-Hall). The syringe disclosed in the patent has the usual barrel, plunger and needle, however, an additional sleeve is slidable over the barrel. The primary purpose of the sleeve is to provide a means for determining precisely the depth of penetration of the needle 6, in FIG. 1 of the patent, into human tissue. The patentee clearly had in mind safety, i.e., minimizing undesired puncture wounds by the users. However, the patent does not address the problem of the security of the sleeve when it is positioned over the needle subsequent to the use of the syringe. 
     There remains a need for a disposable syringe assembly having a safety cover which, once locked in position, cannot, without the use of considerable force, be removed from its locked position. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the disadvantages of the prior art identified above by providing locking means between an auxiliary sleeve and the barrel of the syringe for securely locking the sleeve in a fully extended position in which the needle is fully covered to prevent injury to a user by the needle. More specifically, the invention provides a syringe comprising: a barrel having an hypodermic needle mounted on one end thereof, a piston mounted for movement longitudinally within the barrel and connected to a handle that projects from the opposite end of said barrel; wherein said barrel is tubular and at least partially transparent in a portion that extends over the major part of its length; a tubular sleeve dimensioned to fit over the barrel, and guided to slide axially thereon between an extended position wherein it projects from said one end of the barrel beyond the tip of said needle and a retracted position wherein it coaxially surrounds said barrel and exposes said needle for use, said sleeve having a length that corresponds to that of said barrel and comprising a tubular wall that is at least partially transparent such that in said retracted position it affords unimpeded visibility of substantially the entire length of said transparent portion of said barrel; retaining means to secure said sleeve when in the retracted position against axial movement relative to said barrel; and locking means for securely and non-releasably locking the sleeve in said extended position, in which position the needle is fully covered to prevent injury to a user by the needle. 
     The retaining means retains the sleeve over the barrel to prevent inadvertent displacement of the sleeve so that the clear central portion of the sleeve provides unimpeded viewing of the barrel up to and during the time of use. After use the sleeve is moved to its extended position and the locking means are engaged to securely lock the sleeve in this position. 
     Preferably, a screw-on cap is provided to place over the end of the sleeve once it has been fully extended and locked in position. 
     The invention provides a cheaply constructed, disposable syringe that materially improves safety in that a deliberate and almost self-destructive act would be required before the needle could contaminate the user. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, which illustrate embodiments of the invention; 
     FIG. 1 is a side elevation of a syringe according to the invention; 
     FIG. 2 is a view similar to FIG. 1 with its protective sheath extended; 
     FIG. 3 is an illustration of the syringe with the sheath separated therefrom, and; 
     FIG. 4 is a section on the line 4--4 of FIG. 2. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In the drawings, 10 is a syringe having a transparent cylindrical barrel 11, a plunger 12 bearing a piston 15, and a needle 13. Indicia 14 on the barrel 11 indicate, by reference to the end of the piston 15, the amount of fluid drawn into the syringe. 
     A tubular cylindrical sleeve 16 is dimensioned to fit closely over the barrel 11 so that in its initial position it covers the barrel 11 entirely, allowing the needle 13 to project beyond the end of the sleeve, that is, to the left in FIG. 1. The sleeve 16 is transparent and is of substantially uniform thickness over the major portion of its length so that when the sleeve 16 is in its initial, retracted position as shown in FIG. 1, the barrel 11 and its contents as well as the indicia 14 are clearly visible over their entire extent. The syringe is provided, as packaged, with a standard smaller sleeve 17 of the type known in the art which, in the case of the present invention, is removed prior to use and discarded, since it need not be used again. Barrel 11 and sleeve 16 are formed from a suitable plastic material. 
     The sleeve 16 is secured prior to use by the screw threads at 24 on the outside of the barrel 11, and matching screw threads 25 on the inside of the distal end of the sleeve. The screw threads 24 are formed, in a manner known in the art, such that a quarter-turn of the sleeve 16 in relation to the barrel 11 releases the sleeve 16 for axial movement along the barrel 11. In FIG. 2, the sleeve 16 is shown fully extended to a position where it entirely covers the needle 13, and it is impossible to make accidental physical contact with the needle. Preferably, a cap 8 is provided having external threads 26 matching the threads 25. The cap is packaged with the syringe 10 and used to ensure the safety of the assembly after use. 
     Illustrated on the outside of the barrel 11 is a formation, in the plastic material from which the barrel is formed, of a feature critical to the invention, i.e., the aforementioned secure locking means. Formed on the inside of the sleeve 16 is a protrusion 18 that extends radially inwardly a short distance corresponding to a groove 19 (FIG. 2) formed in the outside of the barrel 11. The groove 19 extends axially along the barrel, and at its right hand or inner end extends circumferentially as at 30 to permit the aforementioned quarter turn locking of the sleeve in the rest position via the threads 24,25. This lateral extension 30 of the groove 19 therefore extends over a quarter, or slightly more than a quarter, of the circumference of the barrel 11. The depth of the lateral extension 30 of groove 19 is such that the protrusion 18 will enter and form a tight fitting frictional engagement therewith. The protrusion will thus be frictionally held in the extension 30 and requires substantial force to be applied to disengage it and move it into alignment with the groove 19. This frictional engagement between the extension 30 and the protrusion 18 provides a further locking means to retain the sleeve 16 in the retracted position. At the opposite or outer end of the groove 11, a ramp 31 is formed extending in depth from its maximum at the groove 19 radially outwardly to a recess 32 corresponding in size and shape to the protrusion 18. Thus, once the sleeve has been rotated a quarter turn freeing it from the threads 24 and freeing it from the portion 30 of the groove, the sleeve can be pulled along the barrel with the protrusion 18 engaged in the groove 19 to its outermost or left hand position. Rotation of the sleeve 16 then causes the protrusion 18 to ride resiliently up the ramp 31 (this movement being accommodated by flexure of the sleeve 16 and/or the barrel 11) and drop into the recess 32, thus locking the sleeve firmly in the disposal position shown in FIG. 2. It will be appreciated that the sleeve 16 cannot be removed from the locked position without considerable force applied either torsionally or axially and as a consequence the user is protected, as are, for example, cleaning staff. The cap 8 is subsequently screwed into position in the barrel to complete the protective covering. 
     It will also be understood that while the invention is primarily intended for use with a syringe, other assemblies having percutaneous needles can be equipped in like manner with protective sleeves.